The settings for Background can be given as follows to apply separately to successive items in the control:

{s1,s2,…,sn}

use s1 through sn, then use defaults

{{c}}

use c in all cases

{{c1,c2}}

alternate between c1 and c2

{{c1,c2,…}}

cycle through all ci

{s,{c}}

use s, then repeatedly use c

{s1,{c},sn}

use s1, then repeatedly use c, but use sn at the end

{s1,s2,…,{c1,c2,…},sm,…,sn}

use the first sequence of si at the beginning, then cyclically use the ci, then use the last sequence of si at the end

{s1,s2,…,{},sm,…,sn}

use the first sequence of si at the beginning and the last sequence at the end

{i1->v1,i2->v2,…}

specify what to use at positions ik

{spec,rules}

use rules to override specifications in spec

With settings of the form {s1,s2,…,{…},sm,…,sn}, if there are more si specified than items across the list picker, si from the beginning are used for the first items, and ones from the end are used for the last items.